When the Minneapolis Interstate 35W bridge collapsed on Aug. 1, 2007, sending 88 vehicles and hundreds of people into the Mississippi River - 13 were killed and 100 injured - nobody, including state bridge inspectors, foresaw such a catastrophic event.
The collapse was a sad reminder of the aging U.S. infrastructure, which needs a $1.6 trillion overhaul over the next five years, according to the American Society of Civil Engineers. Its report graded the U.S. infrastructure as a D in 2005, down from a D+ in 2001. Bridges earned a cumulative C.
U.S. bridges are a pressing concern because stress loads have increased substantially due to a spike in traffic congestion. Since the Minneapolis I-35W bridge was built in 1967, the number of vehicles using the bridge had tripled, according to state documents. Also, truck weight limits have increased nationally, according to the U.S. Department of Transportation (DOT).
The I-35W bridge had been diligently inspected since 1993, and it always passed. Although state officials knew the bridge needed repairs, they had no idea the bridge was in danger of collapse. In January, the National Transportation Safety Board (NTSB) announced the bridge failed due to a design flaw in the gusset plates that connected steel beams; the gusset plates were only half the thickness they should have been. The NTSB investigation found no evidence that cracking or corrosion played a role in the collapse. However, the tragedy had already drawn much-needed attention to the problem of corrosion in aging bridges.
The Minnesota Department of Transportation (Mn/DOT) knew of some risks beforehand: Prior to the accident, Mn/DOT was so concerned about the bridge's structural deficiencies that officials considered replacing it. Other proposed fixes included bolting steel plates to the bridge to prevent stressed areas from cracking. Mn/DOT officials, though, ultimately decided that only more frequent visual inspections were required.
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